An optical communication technique has been required to strictly control a dispersion characteristic for meeting a demand of increasing communication capacity primarily needed for a wavelength division multiplexing in recent years. For this reason, an optical fiber cable has also been required to control a polarization dispersion characteristic.
An optical fiber ideally needs to be completely round in cross section, however, the cross section of the optical fiber substantially includes all sorts of asymmetry such as deviation from a complete round and eccentricity of the circle. The non-circularity of the optical fiber results from production facilities and production conditions, so that the non-circularity tends to continue in the longitudinal direction as well as in a cross section of the optical fiber. The propagation of light through the optical fiber with non-circularity produces a difference in propagation velocity between the X and Y polarization mode being the propagation mode of the light to cause dispersion. This is known as a polarization mode dispersion (PMD).
For the polarization mode dispersion of an optical fiber, the following Patent Documents have proposed an optical fiber and a method of producing the same in which an optical fiber preform is guided by a guide roller periodically swinging at the time of drawing an optical fiber matrix to impart a predetermined torsion to the optical fiber to preclude non-circularity inherent in the cross section of the optical fiber from continuing in the longitudinal direction to equalize the X polarization mode with the Y polarization mode in propagation velocity, thereby reducing the polarization mode dispersion (refer to Patent Document 1, Japanese Patent Application Laid-Open No. H06-171970; Patent Document 2, Japanese Patent Application Laid-Open No. H08-295528; and Patent Document 3, U.S. Pat. No. 5,822,487).
A typical optical fiber ribbon includes a plurality of optical fibers arranged in parallel and a ribbon resin for coating the optical fibers. The optical fiber includes a glass fiber made of silica glass, primary coating layer and secondary coating layer. The structure of the optical fiber ribbon is such that a plurality of optical fibers are arranged in parallel and integrated with the optical fibers contacted or not contacted with one another and collectively covered with a ribbon coating layer.
The optical fiber ribbon is thus structured to form a structural characteristic in which a stress exerting on each optical fiber is different according to a position where each optical fiber is arranged because the optical fiber ribbon is asymmetric in cross section in the thickness and width direction. That is to say, in the optical fiber ribbon formed such that a plurality of optical fibers are arranged in parallel and the periphery thereof is integrated with a coating, each optical fiber is subjected to stress from the coating formed at the production process of the optical fiber ribbon at a position where each optical fiber is arranged, so that an optical fiber arranged on an inner side the ribbon is different in strength and direction of stress exerted thereon from one arranged on an outer side of the ribbon.
An asymmetry in the cross section of the optical fiber ribbon continuing in the longitudinal direction and a different stress exerted on each optical fiber widen a difference in the polarization mode dispersion resulting from the stress between the optical fibers, which tends to deteriorate the polarization mode dispersion in the optical fiber ribbon and the optical fiber cable aggregating the optical fiber ribbons.
In contrast to the foregoing structure, an attempt has been made in a Non-Patent Document 1 in which a stress exerted on the glass fiber is estimated from the ribbon resin of the optical fiber ribbon and a primary and a secondary coating layer provided on the periphery of the glass fiber, and double refraction of each optical fiber is estimated based on obtained stress (refer to Non-Patent Document 1, “Stress Distribution in Optical-Fiber Ribbons,” A. Galtarossa et al., IEEE Photonics Technology Letters, Vol. 9, No. 3, March 1997; Non-Patent Document 2, “Effect of Fiber Displacements on Stress Distribution in 8-Fiber Ribbons,” A. Galtarossa et al., ECOC 97, 22-25 Conference Publication No. 448).
According to the above studies, it is reported that double refraction estimated based on estimated stress values well agrees with the tendency in the polarization mode dispersion exhibited by each optical fiber inside the optical fiber ribbon including a plurality of the optical fibers, which shows that a stress inherent in the coating layer for the optical fiber and the coating for the optical fiber ribbon is intimately associated with the polarization mode dispersion.